As we know, the function of the main flow path, manifold, and gate are to transport the plastic melt from the injection molding machine
nozzle to each cavity. Pouring system condensate can indeed be crushed and reused, but the presence of condensate means that injection molding machine productivity is reduced because the material in the pouring system section must also be plasticized in the barrel of the injection molding machine. For smaller parts, the pouring system condensate may account for 50 or more of the actual injection volume.
Main flow path
The main runner can be seen as a continuation of the nozzle passage in the mold. In single cavity molds, the gate where the main runner leads directly to the part is called a direct gate.
The productivity of a single-cavity injection mold is usually determined by the cooling time of the main runner. In addition to providing sufficient cooling to the main runner bushing, the minimum diameter of the inlet on the main runner bushing should be as small as possible and yet be able to fill the cavity at the right time.
However, there is no universally applicable rule here, because the filling of the cavity is dependent on many factors. The main runner should have a 1.5-degree release slope. A larger release slope allows the main runner to be easily released from the main runner bushing, but when the main runner is longer it will result in a larger diameter and therefore require a longer cooling time. The exit diameter of the injection molding machine
nozzle should be 0.5mm smaller than the minimum aperture of the main channel bushing so that a groove will not be formed at the top of the main channel to prevent the release of the main channel condensate.
In a multi-cavity mold, the plastic melt must be injected into each cavity through a manifold located on the parting surface of the mold. The same basic rules that apply to the main flow path also apply to the cross-section of the manifold. An additional factor that must be considered is that the cross-section of the manifold is also a function of its length since it can be assumed that the increase in pressure loss in the manifold is at least proportional to the length of the manifold.
In most cases, the pressure loss will be greater because its cross-section decreases due to the solidification of the plastic melt along the runner wall, and the pressure loss will be greater the farther away from the main flow channel. In addition, the main flow path and manifold system means loss of raw material and wasted plasticization of the injection molding machine
, so the manifold should be designed as short as possible and the cross-section should be as small as possible. The length of the manifold is determined by the number of cavities in the mold and the geometric arrangement of each cavity.
The shape of manifold cross-section
Because the surface area of the manifold with a circular cross-section is the smallest, the heat loss relative to the cross-section of the manifold is the least, so the manifold with a circular cross section should be used as much as possible. Since the melt in the center of the manifold with a circular cross-section is the last to solidify, the melt can flow along the center of the manifold with a circular cross-section for the longest distance under the action of holding pressure. Thus, the gate (the section between the manifold and the cavity) should be designed in such a way that the melt enters the cavity through the gate from the center of the manifold with a circular or rectangular section.
At the minimum section of the runner, the steel around the gate is locally heated due to the flow friction of the plastic melt, so that the melt can continue to be injected into the cavity in a longer cycle period before the gate solidifies under the action of holding pressure, to play a complementary shrinkage role.
When there must be movement between the smooth surface and the manifold, a manifold with a circular cross-section cannot be used. In this case, a manifold with semi-circular grooves can be used. The advantage of this shape is that the manifold only needs to be machined on one side of the template. However, when the semi-circular slot manifold has the same radius of curvature as the diameter of the shaped manifold, the semi-circular slot manifold holds more than 12.5 more raw materials than the circular manifold.
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